Planets Jupiter, Venus and Mercury, the dynamic threesome we've been communing with near the west-northwest horizon in the evening twilight, is about to become a twosome. Tonight, the trio makes a brief appearance from about 9 to 9:20 and may be seen for a shorter interval for several more days.

An unobstructed view of the horizon is of the essence. The planets form a diagonal line. Mercury, -.32 magnitude (m), is above its brilliant sister Venus (-3.9m); Jupiter (-1.91m) below. Observe that the smaller the magnitude number the brighter the celestial body.

Venus' larger negative number informs us that it is the brightest. Therefore, it will be visible first of the bunch, solo, in the sunset glow at about 8:50. Jupiter appears next and then Mercury, both pale in comparison to Venus. Jupiter sets first, at 9:24 this evening, an hour after sundown.

By next Friday, Jupiter will slip below the horizon only half an hour after the sun; consequently, its reflected light may be overcome by the sunset radiance and it disappears from view. On the 19th, Jupiter will be in conjunction with the sun: the great planet's orbit takes it to the far side of our star.

As twilight deepens, four bright stars form a wide arc above the planetary trio. Known as the Spring Arch, the stars are: Capella (0.06m), to the right and above the planets, Castor (1.56m) and Pollux (1.15m) high above and, on the left, Procyon (0.37m).

The Spring Arch is an asterism, a star pattern that is easily recognizable, not one of the official constellations, and is often composed of stars that belong to several different constellations. Capella is the brightest star in Auriga, The Charioteer. Castor and Pollux are the head stars of Gemini, The Twins, and Procyon is the brightest star in Canis Minor, The Little Dog.

To contact Judy Isacoff go to: www.naturesturn.org.